A radiation type heat dissipator for use in a plasma engine is formed of a refractory metal layer upon which there is deposited a radiation emissive coating made of a high emissivity material such as zirconium diboride. The radiation emissive coating has a surface emissivity coefficient substantially greater than the emissivity coefficient of the refractory metal and thereby enhances the optical radiating efficiency of the heat dissipator

Deininger, William D.

King, David Q.

English

NASA Technical Reports Server

United States Patent 1191 p i ]  Patent Number: 4,928,027 
Deininger et al. [45] Date of Patent: May 22, 1990 
HIGH TEMPERATURE REFRACTORY 
MEMBER WITH RADIATION EMISSIVE 
OVERCOAT 
Inventors: William D. Deininger, Pasadena; 
David Q. King, Sunland, both of 
Calif. 
The United States of America as 
represented by the administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
Assignee: 
Appl. No.: 87,376 
Filed: Aug. 20,1987 
Int. Cl.4 ............................................. HOZK 44/10 
U.S. Cl. ...................................................... 310/11 
Field of Search .......................................... 310/11 
References Cited 
U.S. PATENT DOCUMENTS 
OTHER PUBLICATIONS 
W. Deininger et al.; AIAA-86-1510; Development of 
an Arcjet Nuclear Electric Propulsion System for a 
1993 Flight Demonstration (p. 5); Jun. 1986, AIAA- 
/ASME/SAE/ASEE 22nd Joint Propulsion Confer- 
ence. 
D. Q. King et al.; AIAA-87-1020; Design and Opera- 
tion of a 100 KW Subscale MPD Engine (p. 2); May 
1987; AIAA/DGLR/JSASS International Electric 
Propulsion Conference. 
Primary Examiner-Patrick R. Salce 
Assistant Examiner-Judson H. Jones 
Attorney, Agent, or Firm-Thomas H. Jones; John R. 
Manning; Charles E. B. Glenn 
1571 ABSTRACT 
A radiation type heat dissipator for use in a plasma 
engine is formed of a refractory metal layer upon which 
there is deposited a radiation emissive coating made of 
3,430,082 2/1969 Yerouchalmi ......................... 310/1 1 
4,128,776 12/1978 Boquist et al. 310/11 
4,173,187 11/1979 Steverding .......................... 252/478 
4,180,484 12/1979 Marchant .............................. 310/11 
4,268,765 5/1981 Hoover, Jr. ........................... 310/11 
a high emissivity material such as zirconium-diboride. 
. 4,093,879 6/1978 Marchant et d. .................... 310/11 The radiation emissive coating has a surface emissivity 
the optical radiating efficiency of the heat dissipator. 
........................ coefficient substantially greater than the emissivility 
coefficient Ofthe refractory metal and thereby enhances 
4,218,629 8/1980 Kayu Kawa et al. ................ 310/11 
4,447,748 5/1984 Young et al. ......................... 310/11 7 Claims, 2 Drawing Sheets 
i" 
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https://ntrs.nasa.gov/search.jsp?R=19910005176 2020-03-24T06:57:09+00:00Z
U.S. 
4,928,027 
1 2 
Electrons emitted from the cathode 16 are collected 
at a collecting area 14a on an inner surface of the anode 
14 which defines part of the expansion nozzle 20. Be- 
cause of the high temperatures involved (looo" C. and 
5 above), the electrodes 14 and 16 of the arcjet engine are 
susceptible to destruction from overheating. A variety 
of factors can lead to undesirable heat build up in local- 
ized portions of the engine. Such heat build up should 
preferably be dissipated as rapidly as possible to avoid 
the danger of any melting of the engine parts. The 
anode 14 in suffers from a phenomenon 
known as "anode spot formation" which is character- 
ked by localized melting of various areas at the inner 
such as used in arcjets and ma@eto- 15 most pronounced at the collecting area 14a where the 
wall. The localized melting causes pitting and ulti- 
mately leads to the complete destruction of the anode 
14. 
Active cooling of exterior surfaces of the anode 14 
with dynamic cooling devices such as heat pipes and the 
problem. Active cooling methods are disadvantageous 
and molybdenum, are used in structural elements that 25 moving a fluid. men such pumping systems 
are exposed to the plasma environment. are incorporated into the design of an engine they add 
is a cross sectional view Of a known wet lo* to the overall mass of the engine and increase the cost of 
The =jet lo has an intake section loa into which an the engine. The present invention provides another 
ionizable plasma fluid 11 such as ammonia, argon, he- 
lium or hydrogen is injected at a first velocity. The 30 
arcjet 10 includes a plasma accelerator section 10b for SUMMARY OF THE INVENTION 
accelerating the plasma fluid 11 to a substantially higher In accordance with the present invention, the optical 
second velocity A tubular anode 14 made of a refrac- radiating capabilities of high temperature members 
tory metal such as thoriated tungsten is Provided in the made of refractory metals such as tungsten, molybde- 
accelerator Section lob COnCentriCdly surrounding a 35 num and thoriated tungsten are enhanced by coating a 
refractory metal cathode 16 which iS also composed Of surface of such members with a heat tolerant, high 
a refractory metal such as thoriated tungsten. An insula- emissivity material such as zirconium diboride (ZrBz). 
tive Propellant injector 15, made of a high temperature An ideal black body radiator has a spectral emi,ssivity 
dielectric such as boron nitride, separates the cathode coefficient of 1.00. Known refractory metals have spec- 
16 from the anode 14. The anode 14 is fitted into a 40 tral emissivity values of approximately 0.2-0.4 when 
SkhtlY tapered body 12 made of a refractory metal subjected to temperatures in the range of 1200" C. to 
such as molybdenum. A high voltage +HV is applied 2500' C. This limits their ability to function as radiative 
across a space between the anode 14 and cathode 16 to surfaces for expelling heat from high temperature por- 
generate an electric arc 17. The generated arc 17 inter- tions of devices such as a plasma engine. It has been 
acts with the flowing plasma fluid 11 to heat the fluid to 45 found that a layer of a heat tolerant, high emissivity 
temperatures in excess Of 1OoO" c. and to ionize the material such as zirconium diboide can be deposited 
fluid. onto exterior surfaces of such refractory metals to in- 
The plasma fluid 11 is injected tangentially in a spiral- crease the spectral emissivity coefficients of high tem- 
like fashion into a plenum chamber 18 of the device to perature members made from these refractory metals to 
create a vortex lla about the cathode 16. The vortex 50 values on the order of 0.80 or higher. The improved 
lla is directed into a narrow constiktion area 19 of the emissivity makes possible a highly efficient heat radiator 
accelerator section 10b wherein the plasma fluid i s  SU- that can be used in numerous applications including the 
per-heated by the electric arc 17. The plasma fluid is radiation cooling of plasma engines. 
then accelerated (primarily by thermal expansion) when 
HIGH TEMPERATURE REFRACTORY MEMBER 
WITH RADIATION EMISSIW OVERCOAT 
BACKGROUND OF THE INVENTION 
1. Origin of the Invention 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Public Law 96-517 (35 USC 
202) in which the Contractor has elected not to retain 
title. 
2. Field of the Invention 
The Present invf.Xltion is directed to high temperature surface of the anode. Anode spot formation is often 
Plasmadynamic (MPD) More it is electric arc 17 meets with the expansion nozzle's inner 
structural 
directed to a method for cooling such members. 
3. Description of the Prior Art 
High temperature plasmas such as produced in mag- 
netohydrodynamic generators mm)* magnetoplas- 20 
madynamic WPD) thrusters and arcjets are highly 
such as high temperature ceramics and high 
coflosive to most nmXk&* Consequent1y? refractory like has been proposed as a solution for the overheating 
melting point metals, for tungsten because they require complicated pumping systems for 
for solving the overheating problem. 
:& -"----- rt .  _-.._ t. -- ----_" L- ---- 1- ?n - r6a-  A--.:-- r r  BRIEF DESCRIPTION OF THE DRAWINGS 
4,928,027 
3 4 
the outer surface of a refractory metal. Preferably, the anode of the coated engine 10 operated on average 
high emissivity overcoat is selected to have a coefficient approximately 120" C. cooler than the anode of the 
of thermal expansion approximately equal to that of the uncoated engine 10. The ZrB2 coated engine 10 was 
refractory metal over a predetermined range of operat- operated to power levels as high as 37.0kW without 
ing temperatures. 5 indications of short term failure. The outer surface 
In high temperature environments such as that of the ZrBz coating of the test engine 10' initially had a matte 
above described plasma (arcjet) engine, refractory met- gray appearance which took on a yellowish coloring 
als including for example, tungsten, thoriated tungsten, after engine operation. The reason for this color trans- 
and molybdenum, are used to form conductive elec- formation is not yet fully understood. 
trodes. The electrodes interact electrically with a super- 10 It will be appreciated that materials other than zirco- 
heated plasma fluid of the engine to generate propulsive nium diboride can be deposited on the surface of a re- 
forces. The cathode 16 of FIG. 1 for example, is typi- fractory member to improve the heat radiating capabili- 
cally made of a thoriated tungsten (WTh) having ap- ties of such a member. The high emissivity overcoat 
proximately 80-98 wt % tungsten and 20-2 wt % tho- may be deposited on the outer surface of a refractory 
rium. Thorium is added to the tungsten in order to 15 member by various known deposition methods includ- 
increase the electron e.mission capabilities of the cath- ing for example, sputtering, ion implantation, chemical 
ode at high operating temperatures. The anode 14 is also conversion of the member's surface, thermal spraying, 
typically made of thoriated tungsten while the body 12 impact plating, chemical vapor deposition, electroplat- 
is made of molybdenum. Titanium is another refractory ing, electroless plating and so forth. In some cases, an 
metal often found in such structures. These materials 20 intermediate layer made of a suitable material for ther- 
have relatively low coefficients of spectral emissivity. mally coupling the high emissivity overcoat to the sur- 
The spectral emissivity coefficient of pure tungsten for face of the refractory member may be required to over- 
example, varies between 0.15 and 0.45 depending on come incompatabilities in adhesion properties, differ- 
factors such as temperature, surface finish and the ences of thermal expansion characteristics, to prevent 
wavelength of the radiated energy. The high emissivity 25 undesirable chemical interactions, and so on. The fol- 
overcoat of the present invention improves the radia- lowing table lists her high emissivity materials that may 
tive properties of members having such low emissivity be used as outer surface coatings in accordance with the 
materials. For example, for an overcoat of zirconium invention. This list is, of course, not to be taken in a 
diboride, the emissivity coefficient is greater than 0.8. limiting sense. The range of emissivity coefficient ob- 
The results of comparative tests conducted with a 30 served for several materials are indicated in parenthesis. 
baseline uncoated arcjet 10 and a coated arcjet 10 will 
be described by referring to FIGS. 2 and 3. FIG. 2 
shows a coated portion of the arcjet 10 with parts simi- 
CARBIDES lar to those of FIG. 1 denoted by primed reference 
Tungsten Carbide 
Titanium Carbide numerals. A layer 21 of zirconium diboride (ZrB2) hav- 35 
ing a thicknew of annronimntelv 0 5 J  nln inrh Waf -1 . - . . .  
4,92 8,027 
5 6 
in plasma decelerators such as found in MHD genera- 
tors wherein a high velocity plasma flow is decelerated 
to produce electromagnetic power. High emissivity 
coatings may further be used for other high temperature 
applications that utilize low emissivity refractory mate- 5 
rials such as tugsten and molybdenum. The selection 
of a Particular high emissivity material that is to be 
coupled to a low emissivity base member is of course 
predicated upon considerations such as cost, desired 
temperatwe range, the matching of 
coefficients for the coating and the refractory substrate, 
and the ease with which a Particular Coating material 
can be deposited on the refractory substrate. As such, 
the scope of the invention should not be limited to the 
particular embodiment described above but should 15 
rather be defined by the appended claims and equiva- 
lents thereof. 
We claim: 
1. A heat radiator for radiating heat generated by 
high temperature plasma in a plasma accelerator/decel- 20 
erator having an interior portion in contact with the 
plasma and an exterior portion not in contact with the 
and a coefiicient of thermal expansion substantially 
equal to that of said refractory material. 
2. A heat radiator according to claim 1 wherein the 
refractory material includes a refractory metal. 
3. A heat radiator according to claim 2 wherein the 
refractory metal includes a metal selected from the 
group consisting of tungsten, molybdenum, titanium, 
and thoriated tungsten. 
4. A heat radiator according to claim 3 wherein the 
expansion lo refractory material includes the refractory metal of 
. 
thofiated tungsten. 
second layer has a 
5. A heat radiator according to claim 4 wherein the 
in the range 0.005-0.010 
-L 
. .  
mcn. 
comprising: 
6. A radiation cooled plasma accelerator/decelerator 
plasma fluid inlet means for receiving a plasma fluid 
plasma fluid output means for discharging the plasma 
electrode means for interacting electrically with the 
moving at a frst velocity; 
fluid at a second velocity; and 
nlacma fliiirl tn rhnnup the vplnritv nf the nlacma 


High temperature refractory member with radiation emissive overcoat

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